The role of advection and turbulent mixing in the vertical distribution of phytoplankton T. Serra a, * , T. Granata b , J. Colomer a , A. Stips c , F. Møhlenberg d , X. Casamitjana a a Department of Physics, University of Girona, Campus Montilivi, E-17071 Girona, Spain b Ecological Engineering, CEEGS, Ohio State University, 470 Hitchcock Hall, 2070 Neil Avenue, Columbus, OH 43210-1275, USA c JRC-SAI/ME Unit, Via E. Fermi- T.P. 690, I-21020 Ispra, Italy d DK1, Agern Alle 11, DK-2970 Hoersholm, Denmark Received 28 June 2001; received in revised form 12 December 2001; accepted 12 December 2001 Abstract The purpose of this study is to analyse the role of the flow field on the horizontal and vertical distributions of different phytoplankton populations thriving in the water column of a shallow coastal ecosystem. Two extreme flow conditions are illustrated. The first was a low energetic flow, under calm meteorological conditions and a stratified temperature of the water column. The second flow, coincident with the passage of a storm front, was more energetic resulting in increased mixing that homogenized the temperature in the whole water column. Although the mixing level homogenized the temperature of the water column in the high-energy period, it was not enough to homogenize the temperature in the low-energy period. In contrast, in both periods, the mixing level was enough to homogenize the vertical distribution of particles. A decrease in the concentration of particles from the calm period to the high-energy period was attributed to an advection event with warmer water of lower plankton concentration that resulted in a decrease of the total concentration of suspended particles in the water column. Data are used to test a model of plankton mixing proposed by Ruiz et al. (J. Plankton Res., 18 (1996) 1727). Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: Advection; Microstructure; Mixing; Eddy diffusivity; Potential density; Particle concentration; Mean current 1. Introduction It is known that different phytoplankton groups occupy different niches in the water column depending on their sizes (Tremblay, Legendre, & Therriault, 1997). Small phytoplankton cells (<5 lm) tend to be ubiquitous in the water column, whereas large phytoplankton cells thrive in a narrow range depending on the hydrodynamic conditions. Even though it is not resolved, this difference in the phytoplankton behaviour has been attributed to several factors, such as the sinking rate of phytoplank- ton, their photosynthetic efficiency, the uptake of nutrients, advection and grazing, coagulation, among others. Large cells are susceptible to sinking or grazing by small herbivores that do not influence small cells. The export of larger phytoplankton from the surface to the bottom is crucial in order to explain the transport of carbon through the water column. It is generally well accepted that there is a strong relationship between the vertical mixing of the water column and the distribution of particles within it. Usually, high levels of mixing in the water column are associated with homogeneous distribution of particles. Even though Ryther and Hulburt (1960) and Ignatiaded (1979) showed that particle stratification can occur even when fluid properties appear well mixed, both studies had low vertical resolution of physical and biological data. Other studies, based on vertical microstructure measurements of the fluorescence (Cowles & Desiderio, 1993; Cowles, Moum, Desiderio, & Neuer, 1993; Ruiz, 1996; Ruiz, Garcı´a, & Rodrı´guez, 1996), suggested the presence of microstructures in the mixed layer, which might be caused by a photo-adaptative response to the light gradient or also by the energy dissipation level over the water column. * Corresponding author. E-mail address: teresa.serra@udg.es (T. Serra). Estuarine, Coastal and Shelf Science 56 (2003) 53–62 0272-7714/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0272-7714(02)00120-8